1. Field of the Invention
The present invention relates to a new device for the continuous casting of steel to obtain an end product with a mixed chemical composition, i.e. including, on one hand, the base steel and, on the other hand, the base steel alloyed with elements added during casting.
The invention also relates to a method implemented by the device.
2. Description of the Prior Art
The continuous casting technique for steel is well known. It essentially consists in supplying molten steel from a tundish into a cooled mould made of copper or of a copper alloy, referred to as a continuous casting mould, the latter having a opening at its lower end, and in extracting from it a continuous and partially solidified ingot through this opening.
In general, the molten steel is introduced into the casting mould by means of at least one nozzle, i.e. a generally tubular element arranged between the tundish and the casting mould. The lower end of the nozzle is usually provided with one or two outlet orifices located in the axis of the nozzle or laterally. The lower end of the nozzle emerges below the top level of liquid steel present in the casting mould.
The prior art also includes nozzles intended to ensure better cooling of the superheated liquid steel coming from the tundish. The purpose is to obtain pasty steel at the inlet of the casting mould. In particular, these nozzles may have a heat exchanger formed by a water-cooled copper tube or even a deflector or dome. The purpose of the latter is to force the superheated steel to trickle along the walls of the nozzle in a thin layer in order to increase the surface area for heat exchange. This technique is referred to as “hollowjet casting”.
Moreover, it is common practice to inject inert gas, such as argon, at the level of inlet of the molten steel in the nozzle with in order to avoiding steel oxidation and preventing accidental blockage, especially through the formation of alumina. The technique of hollow-jet casting allows, in particular, reducing the risk of blockage of the gas supply compared with the case where the inlet orifice of the latter is in direct contact with the liquid steel introduced into the nozzle. Thus, one known technique is to inject the inert gas, such as argon, inside the hollow jet. It is also possible to inject a certain amount of finely divided material into the hollow jet using a non-oxidising gas under a pressure slightly above atmospheric pressure as a vector in order to prevent any entry of air. This material is an alloying metal or a ceramic, for example. The purpose is to obtain a metallic alloy or a composite, depending on the case.
Nowadays, continuous casting of products based on steel with a mixed or bi-component chemical composition has raised a lot of interests in a significant number of specific applications, both for long and flat products. The term bi-component refers to products in which the chemical composition of the steel differs depending on the location in the product that is investigated. For Example, the composition can be different in the skin as compared with the core of the product.
In particular, such a technique can be used, for example:                to increase the quality of the surface treatment of the products. In the case of galvanisation, for example, it is desirable to reduce the silicon content in the vicinity of the slab surface in order to improve the suitability of the rolled products for galvanisation;        to enhance castability, for example, in the case of peritectic steels, whose carbon content is in the order of 0.1-0.15% and which are particularly difficult to cast, when modification of the carbon content in the vicinity of the surface is desired;        to cast products whose mechanical properties vary according to thickness, such as a high strength at the surface and high ductility at the core, for instance.        
Processes to obtain metal parts, in particular, steel parts with a mixed chemical composition, such as rolling cylinders, wearing parts, etc., are known in the field of casting.
However, the prior art does not include a simple device to obtain mixed-composition steels by continuous casting. The use of at least two tundishes in parallel, each provided with its own nozzle and acting simultaneously to cast mixed-composition steels into the same casting mould, is known. However, such an arrangement proves very complex and very costly on an industrial scale. Another way of obtaining a bi-component steel, for example, by introducing a metal sheet into the ingot during casting, has been envisioned. But this again proves to be not very practical.